1. Field of the Invention
The invention relates to toners for developing electrostatically charged images formed in an electrophotographic method, an electrostatic printing method, an electrostatic recording method or the like.
2. Description of the State of the Art
A process for developing an electrostatically charged image is that electrically charged fine particles are attracted by means of electrostatic magnetism to adhere them to the surface of an electrostatically charged image support so that the electrostatically charged image can be visualized.
There are two concrete methods for performing such a developing process as mentioned above, i.e., a wet type developing method using a liquid developer in which pigments or dyes are finely dispersed in an insulating organic liquid; and a dry type developing method, such as a cascade method, a fur brush method, a magnetic brush method, an impression method, a powder-cloud method, or the like, which uses a powder developer comprising toners in which a coloring agent such as carbon-black and the like is dispersedly contained in a binder comprising natural or synthetic resins.
An image visualized in a developing process is sometimes fused as it is to a support and is commonly transferred to such a support as a transfer paper and then fused thereto. The toners of this kind are applied not only to a developing process but also to the successive processes, i.e., a transfer process and a fusing process and such toners are therefore required to display such characteristics as an excellent image transferability and an excellent fusibility as well as an excellent developability. Among these characteristics the most strict conditions are required for the fusibility, and accordingly there have been published many literatures and reports on the studies of the improvements on the fusibility of toners and on the results thereof.
Generally speaking, a heat-fusing method is advantageous for fusing a toner image formed in a developing process or a transferred toner image. In these heat-fusing methods, there are a non-contact heat fusing method such as an oven-fusing and the like, and a contact heat-fusing method such as a heat-roller fusing and the like. The contact heat-fusing methods are excellent in the thermal efficiency, and are in particular suitable for a high speed copying apparatus because a rapid fusing is possible in this method. Also in this method, power consumption can be economized because a relatively low heat source may be used, and the miniaturization of a copying apparatus and energy saving can also be provided. Further, there is no danger of fire even if a sheet of paper remains inside of a fusing unit, that is another merit thereof.
Contact heat-fusing methods are the desirable in various aspects as described above, however, there is a problem in the methods that an offset phenomenon will occur. The offset phenomenon stains an image in the manner that a part of toners forming the image is transferred to the surface of a heat-roller when fusing the image and the transferred toners are further transferred to a sheet of transfer paper or the like which is brought in for the next cycle. To prevent the offset phenomenon, there is an effective means in which a cleaning member such as a cleaning roller is brought into contact with a heat-roller to clean up toners adhered to the heat-roller.
However, when providing such cleaning member, the so-called back-stain phenomenon will occur. This back-stain phenomenon is that, when toners piled on a cleaning member are excessively heated, the toners are transferred to a heat-roller and then they stain the surface of an image support such as a transfer paper fed thereafter, and the toners are further transferred to a pressure-contact roller brought into pressure-contact with the heat-roller and thus the back surface of the image support is stained with the toners transferred to the pressure-contact roller.
As the results of investigating the causes of such back-stain phenomenon, it was found that the elasticity of toners being fused will seriously affect to cause this back-stain phenomenon. To be more concrete, a thermoplastic resin of which the principal component is a styrene-acryl copolymer, for example, has so far been used as a binder resin for toners, and the resins of this kind are apt to be transferred because of the less elasticity of the resins when being fused, and therefore, even though the resins are adhered once to a cleaning member, and in the case of using a heater for a heat-roller for example, the resins are transferred to the heat-roller when toner adhered to the cleaning member are heated up, and a back-stain phenomenon is consequently caused.
In contrast with the above, toners of which the elasticity when being fused is so great that a back-stain phenomenon does not occur, are very bad in the fusibility, because the elasticity thereof is still great even when the toners are softened and fused by a heat-roller and also because the wettability thereof to an image support such as transfer paper or the permeability thereof to the gaps between the fibers of paper is small.
The invention has been devised by taking the abovementioned circumstances into consideration.